This project seeks to understand the mechanisms that led to the generation of autoimmune responses to small nuclear ribonucleoproteins (snRNPs). Generation of these responses is closely linked to the pathogenesis of systemic lupus erythematosus, scleroderma, polymyositis, and related syndromes. The work will characterize a newly-identified snRNP known as the Th particle. This snRNP will be defined structurally and analyzed in comparison to the RNase P particle,a snRNP of known function which may have a physical interaction with the TH particle. Finally, the role of the Th and RNase P particles as autoimmunogens will be defined. Thus, this work aims to understand a new aspect of biology--the structure and function of the Th particle --and to provide insight into causative processes for autoimmunity. This work began with the earlier observation that autoantibodies to the Th and RNase P particles occur in patients with scleroderma and possibly other rheumatic diseases. These antibodies were used to identify the RNA components of both particles and to demonstrate the nucleolar location of the Th particle, although the polypeptide components of both snRNPs are as yet undefined. Information about the structure of the Th snRNP and its behavior within cells is now needed to establish its biological function. The present work will characterize the structure of this particle using immunoprecipitation and immunoblotting methods, establish the nucleotide sequence of the Th RNA, and compare structural features of the Th and RNase P snRNPs. Using autoantibodies as probes, clones for cDNAs of the polypeptide component(s) of the Th snRNP will be derived and used to determine the primary amino acid structure of these polypeptides and to define their autoantigenic epitopes. Finally, the autoantibodies will be used to localize the Th snRNP within the nucleolus at both the light and electron microscope level and to detect alterations that this particle undergoes at specific points in the cell cycle. These structural studies will seek common denominators that designate restricted regions of certain polypeptides as autoantigens.